PLANETARY EVOLUTION 



bodies, the inference is unavoidable that the 

 matter which composes them was formerly much 

 more rare and diffused than at present. If we 

 are to infer the sun's past condition from its 

 present condition, we must necessarily suppose 

 that its constituent matter once occupied much 

 more space than at present, " and we are en- 

 titled to suppose that it extended as far as we 

 can trace effects such as it might naturally leave 

 behind it on retiring ; and such the planets are." 

 The abandonment of successive equatorial zones 

 by the shrinking solar nebula follows from 

 known mechanical laws ; and the subsequent 

 breaking up of each zone, and the consolida- 

 tion of its fragments into a planet, are processes 

 which similarly involve none but established 

 dynamical principles. It equally follows, from 

 elementary laws of mechanics, that the planets 

 thus formed would revolve and rotate both in 

 the directions and in the planes in which they 

 are actually observed to revolve and to rotate. 

 There is thus, observes Mr. Mill, nothing gra- 

 tuitous in Laplace's speculation : " it is an ex- 

 ample of legitimate reasoning from a present 

 effect to a possible past cause, according to the 

 known laws of that cause." 



But the evidence in favour of the theory of 



nebular genesis is not restricted to these general 



coincidences between observation and deduction. 



Many striking minor details in the structure of 



261 



